http://www.busybox.net/:
BusyBox combines tiny versions of many common UNIX
utilities into a single small executable. ... BusyBox
provides a fairly complete environment for any small or
embedded system.

Heirloom Documentation Tools:
This package provides troff, nroff, and related utilities to
format manual pages and other documents for output on
terminals and printers. They are portable and enhanced
versions of the utilities released by Sun as part of Solaris,
which are a variant of ditroff, which, in turn, descends to
the historical Unix troff that generated output for the C/A/T
phototypesetter.

MINIX 3:
a new open-source operating system designed to be highly
reliable, flexible, and secure. This new OS is extremely
small, with the part that runs in kernel mode under 4000 lines
of executable code. The parts that run in user mode are
divided into small modules, well insulated from one another.

Nexenta:
NexentaOS is a complete GNU-based open source operating system
built on top of the OpenSolaris kernel and runtime.

PearPC
[PowerPC emulator for Intel IA-32 systems; Darwin, MacOS X,
and Mandrake Linux 9.1 for PowerPC all run on it]